1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a Micro Electro Mechanical System (MEMS) switch, such as a Radio Frequency (RF) switch, fabricated using a MEMS technique, and in particular, to a MEMS switch which is driven by using a piezoelectric element or actuator, and a method of fabricating the same.
2. Description of the Related Art
Recently, a lot of electronic systems, which are used at a high frequency band, are tending to reduce a weight to an ultra lightweight, to decrease a size to an ultra small size, and to enhance a performance in a high level. Accordingly, an ultra small-sized micro switch using a MEMS technique has been actively developed to substitute for a semiconductor switch, such as a field effect transistor (FET) or a PIN diode, which is using till now to control a signal in the electronic systems.
Among RF elements using the MEMS technique, an RF switch is most widely fabricated. The RF switch is used much in an impedance matching circuit or for selectively transmitting a signal in wireless communication terminals and systems of microwave or millimeter wave band.
As driving mechanisms for use in the RF switch using the MEMS technique, there are known driving mechanisms of various types, such as an electromagnetic type, a magnetic type, a piezoelectric type, an electrostatic type, etc.
According to a conventional electrostatic type MEMS switch, when a fixed electrode is applied with a DC voltage, electrification occurs between the fixed electrode and a movable electrode. Accordingly, the movable electrode is led under the influence of an electrostatic force, so that a contact member formed on the movable electrode comes into contact with or move away from a signal line formed on the substrate, thereby switching signal flow.
The conventional MEMS switch, however, uses the electrostatic force generated between the fixed electrode and the movable electrode as described above to switch the signal flow. Thus, there arises a problem that a high driving voltage should be applied to the movable electrode to drive the movable electrode.
Further, the conventional MEMS switch has a different shape according to a position of cell in a wafer in which it is formed. Thus, a gap between the fixed electrode and the movable electrode is not uniform, but different according to the MEMS switches, thereby uniformity in performances of the MEMS switches being deteriorated. Also, the conventional MEMS switch requires a large number of fabrication processes, thereby a productivity being deteriorated.
In addition, the conventional MEMS switch is disadvantageous in that a contact force of the contact member is unstable, and a contact loss is increased as the contact member repeats the switching operation.
FIG. 1 is a top plan view exemplifying a structure of conventional MEMS switch using a piezoelectric actuator.
Referring to FIG. 1, there is illustrated an upward driving type piezoelectric RF MEMS switch 20 using Pb(Zr, Ti)O3 (lead zirconate titanate) (PZT) as a material of the piezoelectric actuator such as a cantilever.
The piezoelectric RF MEMS switch 20 includes a substrate 1 having an RF input signal line 22a and an RF output signal line 22b plated thereon, and a plurality of cantilevers 21a, 21b, 21c, and 21d to support a contact pad 22. The contact pad 22 is located apart from and just below the RF input and output signal lines 22a and 22b. 
The cantilevers 21a, 21b, 21c, and 21d are formed of an upper electrode layer (not shown), a piezoelectric layer (not shown), a lower electrode layer (not shown), and a membrane (not shown), respectively. When the layers of the cantilever 21a, 21b, 21c, and 21d are applied with a DC voltage, the cantilever 21a, 21b, 21c, and 21d are bended upward in a cavity 23a. As a result, the contact pad 22 formed on top ends of the cantilever 21a, 21b, 21c, and 21d comes in contact with the RF input and output signal lines 22a and 22b to interconnect them with each other, thereby transmitting an RF signal.
Such a conventional piezoelectric RF MEMS switch 20 is advantageous in that it is possible to drive the cantilevers with a voltage of less than 3V, e.g., to move the cantilevers having a length of about 100 μm by about 1.8 μm with the voltage of less than 3V, and there is almost no power consumption.
However, according to the conventional piezoelectric RF MEMS switch 20, there are a lot of difficulties in fabrication. More particularly, a fabrication process is complicated. In most of the piezoelectric RF MEMS switches, the piezoelectric layers and the membranes of the cantilevers are formed at a very high temperature. Thus, the piezoelectric layers and the membranes have to be formed prior to coplanar waveguide (CPW) wire lines including the RF signal lines. If the CPW wire lines are formed on the substrate and then the piezoelectric actuators are formed on the CPW wire lines, a metal is diffused from the CPW wire lines, or a silicide is formed, due to a high temperature. Due to such a restriction, as shown in FIG. 1, the piezoelectric RF MEMS switch should be configured, such that the cantilevers 21a, 21b, 21c and 21d are bended upward and the substrate 1 or a separate wafer is installed above the cantilevers 21a, 21b, 21c and 21d to form the CPW wire lines thereon. In this case, a rear surface (undersurface) of the substrate 1 should be unreasonably etched. In the conventional piezoelectric RF MEMS switch 20 shown in FIG. 1, the cantilevers 21a, 21b, 21c and 21d are formed by etching the undersurface of the substrate 1 after the upper surface of the substrate 1 is plated with the RF signal lines 22a and 22b by a plating process.